1. Field of the Invention
The present invention relates generally to welding and, more particularly, concerns a welding apparatus and method for automatically welding a stack of metal tubes together. This type of welding is an essential operation in various applications, including the construction of heavy-duty frames for towable vehicles.
2. Description of the Prior Art
There are various known ways to weld a stack of tubes together. One representative approach is a manual operation involving an initial tack welding operation followed by a permanent metal-inert-gas (MIG) welding operation.
Generally, this approach has two distinct steps. First, an initial series of tack welds are made at various locations along the longitudinal interface (or seam) between each tube in the stack. The purpose of this initial tack welding operation is to provide a bond between the tubes of sufficient strength to keep the tubes in place during the ensuing MIG welding operation. In order to prevent misalignment amongst the tubes as the welds are being made, clamps must be used to bind the tubes in place. Two clamping techniques are known in the art. Either the stack can be completely immobilized by a myriad of clamps placed at various locations along the full length of the stack; or the tubes can be locally secured by two or three clamps. In the former technique, all of the tack weld can be made in a single pass without moving any of the clamps; and, in the latter technique, the clamps must be continuously relocated along the length of the stack throughout the tack welding operation. Both clamping techniques, however, are time consuming and labor intensive.
Second, after the tack welds have been completed, intermittent MIG welds are made along each seam in the stack. In particular, all the seams along one side of the stack are welded during the first pass, after which the stack is turned over, and all of the seams on the reverse side are welded during the second pass. During the first pass, however, the heat imposed by the welds tends to warp the stack. To help alleviate this problem, it is generally preferable to make the welds on the reverse side longer than those on the first side. Frequently, though, the warping caused by the first pass cannot be fully corrected and the stack remains slightly crooked.
Thus, aside from being very labor intensive and time consuming, this manual approach also tends to be rather non-repetitive. Although other approaches have eliminated some of the problems associated with the above-described operation, none have completely automated the procedure nor has any single approach satisfactorily corrected all of the deficiencies.